This double duty move targets your chest, shoulders, and, of course, your abs, as your obliques have to work hard help stabilize your torso in opposition to the weight.
How to do it: Grab one heavy dumbbell and liefaceup on the floor with both knees bent, feet lifted 90-degrees above your hips. Brace your abs in tight and extend arms above your chest, holding onto weight with both hands. Grab the weight with your right hand, and extend both arms out to sides (elbows should be only slightly bent, palms facing up), stopping just before arms touch the floor. Use your core muscles to keep your shoulders and hips pressed into the floor. Keeping arms extended, bring arms back together over chest, pass the weight to your left hand, and repeat. That's one rep. Do up to 3 sets of 10 repetitions.